1. Field of the Invention
This invention relates to the welding of reactive metals such as titanium and zirconium and more particularly to a backing shield for the protection of the root of the weld and the adjacent base metal from the atmosphere during welding.
2. Background Information
In the welding of reactive metals such as titanium and zirconium, inert gas shielding is required to protect the root of the weld and the adjacent base metal during welding. One method is by using a temporary backing bar or ring as shown on page 451 of Welding Handbook, 7th Edition, Vol. 4, American Welding Society, Miami, Fla. 33126. As shown therein, the bar is usually made of copper; is incorporated in the fixtures; and, contains a clearance groove under the joint that is purged of air prior to welding. With this method, the reactive metal must be in contact with the backing bar along the bar's entire length to ensure a uniform weld. Further, the root opening of the joint must be near zero to prevent the arc from impinging on and fusing the reactive metal to the backing bar, contaminating and making brittle the reactive metal usually resulting in a cracked weld. The root side of the weld is the side opposite from where the welding energy is normally applied.
A limitation of backing shields of the backing bar type is that such shields are generally specially designed for the particular joint to be welded, resulting in an expensive fabrication process and a large inventory of appropriate shields. When complex specially fabricated backing shields are used and one is damaged during the welding process, delays in production can result because of the delay in repairing or replacing a complex shield. U.S. Pat. No. 3,450,857 to Webb discloses an inert gas shield and pictorially suggests that it may be used as a backing shield. Generally, in Webb, the inert gas is introduced into one end of a cavity and distributed along the length of the cavity by the volume of the cavity. The exit of gas to the work piece is controlled by a flat sintered metal surface having pores in a specified range. It is suggested that steel wool may be used as a form of baffle to equalize the pressure drop along the cavity. With this device, it is necessary to vary the porosity of the flat sintered metal surface in order to vary the flow rate at different places along the length of the cavity. Inert gas shielding is also used for weld side shielding as shown in U.S. Pat. No. 3,875,364 to Boyett. In Boyett, a weld side shield with a rigid enclosure is attached to the trailing edge of an electrode holder. Within the enclosure is a single perforated tube that extends substantially the length of the enclosure and is open at one end. The perforations allow the inert gas to escape into the enclosure in a direction toward the innermost part of the enclosure. The end of the enclosure nearest the electrode is substantially open to allow the inert gas to exit the enclosure and impinge upon the area of the weld pool. The end of the enclosure furthermost from the weld pool is partially open to allow the weld to pass under the furthermost end. The side of the enclosure adjacent the weld is open. Within the enclosure is a mass of metal fibers that surrounds the perforated metal tube and acts as a diffuser for the inert gas. A screen retains the metal fibers within the enclosure at the side adjacent the weld and at the end adjacent the electrode.
There is a need for a backing shield that can evenly distribute the inert gas along its length before the gas is projected in a randomly diffused manner toward the root of the weld; and, further, there is a need for a flexible backing shield which can be readily assembled into complex shapes and which can have segments easily replaced. Applicants' invention illustrates how these needs are met in an efficient and cost effective way.